“You are what you eat” and other defensive strategies of marine invertebrates, Part I

Before delving into this post’s subject, a reminder that the biology club’s “Saving Nemo” event is on tonight (February 14) at 8:30 pm in the Thirsty Moose Pub. Furthermore, the third installment in this year’s Prince George Reef Tank Tour will be held on Sunday, February 16, noon to 4 pm. Contact Russell Vander Ende (whatcaneyedo@hotmail.com for directions if you are interested.

Portable homes

For all but a small number of very large animals, one of the main challenges in life is to simply avoid becoming somebody else’s dinner. For every organism, something tends to be around that wants to eat you. The ways in which some marine organisms ensure that this is less likely to occur is the topic of this and the next blog. It doesn’t fall clearly under the umbrella of symbiosis, because one of the participant organisms is frequently the dinner of the other. To add insult to injury, some of these organisms actually use some part of their meal ticket to defend themselves against their own predators. Such tactics aren’t necessarily unique to marine animals in that many terrestrial organisms may acquire toxic chemicals from their meal as a means of avoiding predation. Examples include butterflies that sequester defensive compounds from plants (Nishida 2002), and poison dart (Dendrobatidae) and Mantella species poison frogs (Mantellidae) of Madagascar that acquire toxins from the ants they feed on (Clark et al. 2005). In the case of the marine organisms I will describe below, however, we are discussing the use of parts of another organism.
In this post I will discuss an example that is represented in the reef tank, and where predation is only indirectly involved, i.e., the subject of the post is not the predator. These are the hermit crabs (Crustacea: Decapoda: Paguroidea), a group that includes both terrestrial and marine members. Most species have a very soft and vulnerable abdomen, and many are quite small, necessitating some form of protection against predation. They have solved this dilemma by utilizing the shells of suitably sized snails (Mollusca: Gastropoda) to get the protection they need. Hermit crabs cannot make their own shells, so the availability of suitably sized empty shells is frequently a limiting resource that influences behaviours and population dynamics of the crabs. Thus, acquiring a new shell as the old one is out-grown or becomes damaged becomes a very important task. The method that appears to be predominantly employed among marine hermit crabs is described by McLean (1974), and involves taking advantage of predation on suitably sized

snails by predators. Large numbers of hermit crabs accumulate around a predation event, a dominance hierarchy is established among the waiting crabs, and once the newly empty shell has been dropped by the predator, it is investigated and if suitable acquired. Similarly, Small and Thacker (1994) reported that terrestrial hermit crabs were strongly attracted to the the odour of dead conspecifics. Eviction of a smaller crab from a desired shell is also a possibility, as seen in this video http://www.arkive.org/common-hermit-crab/pagurus-bernhardus/video-03.html. When a hermit crab acquires a new shell, the old shell obviously becomes available, leading to a chain reaction, a “shell vacancy chain” (Chase et al. 1988), where smaller crabs move up to a larger size. Some hermit crabs may prey directly on gastropods to acquire shells, a behavior sometimes observed in reef tank environments, but this behavior appears to be rare, if it occurs at all. In fact, Laidre (2011) found that neither a marine nor a terrestrial species of hermit crab could readily access shells unless they were empty and on the surface of the substrate. Even a dead snail in the shell prevented access.

While the shell offers protection, it also is cumbersome, and often leads to awkward situations for the crabs. You can observe this in the reef tank as the crabs often fall off corals and end up upside down, a position where they are potentially vulnerable. It is somewhat comical to watch them trying to right themselves, nervously exiting only far enough to reach the bottom, and rapidly withdrawing at the slightest disturbance.